1. Field of the Invention
This invention relates to a method of treating photoresist materials applied on semiconductor wafer and, more particularly, to a method of treating the photoresist materials employing ultraviolet radiation.
2. Description of the Prior Art
As for the prior-art treatment of a photoresist employing ultraviolet radiation, exposing the photoresist to ultraviolet radiation is utilized in a treatment in which a mask pattern is formed by exposing the photoresist image applied on a semiconductor wafer, in a preliminary cleansing treatment in which organic pollutants sticking on the surface of the semiconductor wafers are dissolved and cleansed away, etc., and recently attention has been paid to the application of this method to particular treatments of photoresists such as baking photoresist materials.
In the manufacture of semiconductor devices, a process of forming a photoresist pattern is executed in the sequence of the application of a photoresist material, prebaking, exposure, development and postbaking, when roughly divided. The postbaking process is a heating process executed for improving the adhesiveness of the photoresist to a semiconductor substrate, heat-resistance, etc. Thereafter, ion implantation, or plasma-etching of a silicon oxide film, a silicon nitride film, an aluminum film, etc., which are formed beforehand on the surface of a semiconductor wafer prior to the application of the photoresist material, is executed by using the photoresist pattern. On the occasion, it is preferable that the photoresist has a high heat-resistance, since the temperature thereof rises in the process of ion implantation, while it is requisite that the photoresist shows a durability not allowing "film erosion" in the processof plasma-etching. However, a photoresist material of high resolution, which has been used in recent years as semiconductor devices are highly integrated and made highly fine, is of a positive type, and the photoresist of this type is generally inferior in the heat-resistance to the one of a negative type.
With a view to enhancing the heat-resistance and plasma-resistance of the photoresist, examinations and studies are made on a method in which a photoresist material is heated gradually to an elevated temperature for a sufficient time in postbaking, and a method in which ultraviolet radiation are applied to a photoresist pattern after development thereof. These method are described in a reference of H. Hiraoka and J. Pacansky: J. Vac. Sci. Tech. 19(1981).
The former method, however, has a fault that it fails to ensure satisfactory heat-resistance and plasma-resistance and requires to expose the photoresist to ultraviolet radiation for a considerably long time. The latter method, on the other hand, has a fault that, although the heat-resistance temperature thereof is raised by exposing the photoresist to ultraviolet radiation, ultraviolet rays do not penetrate to the depth of a film of photoresist material when it is thicker, which results in an insufficient improvement in the heat-resistance of the entire film of photoresist material and requires a considerably long exposure time.
In view of these faults, a method of combination of "heating" with "ultraviolet radiation" has been proposed recently, as is disclosed in Japanese Unexamined Patent Publication ("KOKAI KOHO" in Japan) 60-45247 (U.S. Application No. 497,466) titled "hardening of Photoresist and Apparatus", for instance. This method, however, is unable to meet the demands for a short exposure time when those photoresists is thick. Namely, an organic compound prevailingly used in photoresist materials, reacts readily to oxygen gas or moisture by being exposed to ultraviolet rays. It is known that the photoresist which was made to react with oxygen or moisture is inferior in the heat-resistance and the plasma-resistance.
Accordingly, in the method of treating photoresists employing ultraviolet radiation which meets the demand for improvement in the heat-resistance and the plasma-resistance, the degree of polymerization of the photoresist increases by ultraviolet radiation, therefore the heat-resistance and the plasma-resistance of the photoresist may be improved. The photoresist, however, is oxidized by ultraviolet radiation, therefore photoresist is inferior in the heat-resistance the plasma-resistance. And polymerization of the photoresist is suppressed by oxygen and moisture in the atmosphere or moisture captured in the photoresist, too. Therefore, this is one of causes for being unable to shorten a exposure time.
When a ultraviolet light is applied to the photoresist, however, a gas is generated from the photoresist and this gas causes the formation of bubbles, deformation of a photoresist pattern and deformation of a photoresist film, such as exfoliation, rupture or roughening thereof, thus causing imperfections of a semiconductor element.
As described above, the prior-art methods for particular treatments of the photoresist, such as hardening the photoresist material employing ultraviolet radiation, have left unsettled the problems that a long exposure time for ultraviolet radiation is required for the treatment and that the improvement in the heat-resistance is insufficient in the base portion of the photoresist film when this film is thick, though they have been able to achieve some improvements in the heat-resistance and plasma-resistance. In other words, there has been left unsettled a problem that the whole treatment of the photoresist can not be performed in an organic and effective manner.
And, when a ultraviolet light is applied to the photoresist, a gas is generated from the photoresist, and this gas causes the formation of bubbles, deformation of a photoresist pattern and deformation of a photoresist film, such as exfoliation, rupture or roughening thereof, thus causing imperfections of a semiconductor element.